第三章NMR實(shí)驗(yàn)技術(shù)基礎(chǔ)

1、granmr06.doc第三章 NMR實(shí)驗(yàn)技術(shù)基礎(chǔ)1 NMR儀器如圖，現(xiàn)代超導(dǎo)核磁譜儀的主要組成部分包括：1. 超導(dǎo)磁體Magnet包括Field Lock，Shim Coils 2. 探頭Probe內(nèi)有RF Coils，Gradient Coils 3. 脈沖編程器及射頻放大器4. 接收器5. 數(shù)據(jù)采集及處理計(jì)算機(jī)At the top of the schematic representation（圖示）, you will find the superconducting magnet of the NMR spectrometer. The magnet produces the Bo

2、field necessary for the NMR experiments. Immediately within the bore of the magnet are the shim coils for homogenizing the Bo field（勻場）. Within the shim coils is the probe. The probe contains the RF coils for producing the B1 magnetic field necessary to rotate the spins. The RF coil also detects the

3、 signal from the spins within the sample. The sample is positioned within the RF coil of the probe. Some probes also contain a set of gradient（梯度）coils. These coils produce a gradient in Bo along the X, Y, or Z axis. Gradient coils are used for for gradient enhanced spectroscopy, diffusion, and NMR

4、microscopy experiments. 圖示的頂端是NMR譜儀的超導(dǎo)磁體。它產(chǎn)生NMR實(shí)驗(yàn)必需的Bo場。超導(dǎo)磁體的核心是用于勻場的墊片線圈。其中有探頭，包括產(chǎn)生用于旋轉(zhuǎn)的B1電磁場的RF線圈。RF線圈同樣探測樣品的旋轉(zhuǎn)信號。樣品由此定位。一些探頭中包含一系列的梯度線圈。這些線圈在Bo沿X，Y，Z方向產(chǎn)生梯度場。梯度線圈是被用來產(chǎn)生加強(qiáng)光譜，散射和其他NMR微尺度實(shí)驗(yàn)的梯度的。The heart of the spectrometer is the computer. It controls all of the components of the spectrometer. The R

5、F components under control of the computer are the RF frequency source and pulse programmer. The source produces a sine wave of the desired frequency. The pulse programmer sets the width, and in some cases the shape, of the RF pulses. The RF amplifier （擴(kuò)音器）increases the pulses power from milli Watts

6、 to tens or hundreds of Watts. The computer also controls the gradient pulse programmer which sets the shape and amplitude of gradient fields. The gradient amplifier increases the power of the gradient pulses to a level sufficient to drive the gradient coils. The operator of the spectrometer gives i

7、nput to the computer through a console terminal with a mouse and keyboard. Some spectrometers also have a separate small interface for carrying out some of the more routine procedures on the spectrometer. A pulse sequence is selected and customized from the console terminal. The operator can see spe

8、ctra on a video display located on the console and can make hard copies of spectra using a printer. 頻譜儀的核心是計(jì)算機(jī)。它控制著頻譜儀的所有組件。被計(jì)算機(jī)控制的RF組件是RF頻率源和脈沖程序。源負(fù)責(zé)產(chǎn)生需求頻率的正弦波，脈沖程序則設(shè)置RF脈沖的寬度，某些情況下也設(shè)定形狀。RF放大器把脈沖能量從百萬分之一瓦特放大到幾十甚至幾百瓦特。計(jì)算機(jī)同時控制用來設(shè)置梯度場形狀和振幅的梯度脈沖程序。梯度放大器把梯度脈沖能量提升到可以帶動梯度線圈的層次。操作者通過控制終端輸入計(jì)算機(jī)。一些頻譜儀也有獨(dú)立的界面用以

9、執(zhí)行一些常用的程序。脈沖序列是由控制終端被選擇和控制的。操作者可以在屏幕上看到控制臺的設(shè)置并打印出來。1. 超導(dǎo)磁體MagnetMagnet主要要求：a 高磁場強(qiáng)度,分辨率與B0成正比，而靈敏度與成正比，故750MHz較600MHz的分辨率提高25%,而靈敏度提高40%b 高均勻性,目前可達(dá)10-9c 高穩(wěn)定性The NMR magnet is one of the most expensive components of the nuclear magnetic resonance spectrometer system. Most magnets are of the supercondu

10、cting type. A superconducting magnet has an electromagnet made of superconducting wire. Superconducting wire has a resistance approximately equal to zero when it is cooled to a temperature close to absolute zero (-273.15C or 0 K) by emersing it in liquid helium. Once current is caused to flow in the

11、 coil it will continue to flow for as long as the coil is kept at liquid helium temperatures. (Some losses do occur over time due to the infinitesimally small resistance of the coil. These losses are on the order of a ppm of the main magnetic field per year.) The length of superconducting wire in th

12、e magnet is typically several miles. This wire is wound into a multi-turn solenoid or coil. The coil of wire and cryroshim coils are kept at a temperature of 4.2K by immersing it in liquid helium. The coil and liquid helium are kept in a large dewar. This dewar is typically surrounded by a liquid ni

13、trogen (77.4K) dewar, which acts as a thermal buffer between the room temperature air (293K) and the liquid helium. NMR磁體是整個核磁共振頻譜儀中最貴的組件之一。大多數(shù)磁體是超導(dǎo)的。超導(dǎo)磁體擁有一塊用超導(dǎo)線制作的電磁石。超導(dǎo)材料的性質(zhì)是通過把它浸入液氦中降到接近絕對零度時它將有接近0的電阻。只有一致保持在液氦中的溫度那么開始其中導(dǎo)入的電流將一直運(yùn)行下去。磁體中超導(dǎo)線的長度典型的有幾英里。它被繞成螺線管或線圈。線圈被浸入在保持4.2K的液氦中，液氦則放置在杜瓦瓶中。杜瓦瓶中裝的是

14、液氮，77.4K,作為和外界的隔熱層。The following image is an actual cut-away view of a superconducting magnet. The magnet is supported by three legs, and the concentric（同中心的） nitrogen and helium dewars are supported by stacks coming out of the top of the magnet. A room temperature bore hole extends through the cent

15、er of the assembly. The sample probe and shim coils are located within this bore hole. Also depicted in this picture is the liquid nitrogen level sensor, an electronic assembly for monitoring the liquid nitrogen level. 下面的圖像是超導(dǎo)磁體的橫截面圖。磁體被同中心的液氮和液氦杜瓦瓶包圍。一個室溫通道孔延伸到裝置的中心。樣品管和墊片線圈就位于這個孔中。圖中還顯示了一個液氮層傳感器用

16、來控制液氮層。Going from the outside of the magnet to the inside, we see a vacuum region followed by a liquid nitrogen reservoir. The vacuum region is filled with several layers of a reflective mylar film. The function of the mylar is to reflect thermal photons, and thus diminish heat from entering the mag

17、net. Within the inside wall of the liquid nitrogen reservoir, we see another vacuum filled with some reflective mylar. The liquid helium reservoir comes next. This reservoir houses the superconducting solenoid or coil of wire. 磁體從外到內(nèi)，可以看到在液氮層后存在一個真空區(qū)域。真空區(qū)域是有幾層反射性薄膜組成。其作用是反射熱聲子，從而減少進(jìn)入磁體的熱量。在液氮池的內(nèi)側(cè)有另外

18、一個填充著反射性薄膜的真空區(qū)域，緊接著就是液氮，可以覆蓋超導(dǎo)螺線管或線圈。Taking a closer look at the solenoid it is clear to see the coil and the bore tube extending through the magnet. Field LockIn order to produce a high resolution NMR spectrum of a sample, especially one which requires signal averaging or phase cycling, you need to

19、 have a temporally constant and spatially homogeneous magnetic field. Consistency of the Bo field over time will be discussed here; homogeneity will be discussed in the next section of this chapter. The field strength might vary over time due to aging of the magnet, movement of metal objects near th

20、e magnet, and temperature fluctuations. Here is an example of a one line NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. The field lock can compensate for these variations.The field lock is a separate NMR spectrometer within your spectrometer.

21、 This spectrometer is typically tuned to the deuterium NMR resonance frequency. It constantly monitors the resonance frequency of the deuterium signal and makes minor changes in the Bo magnetic field to keep the resonance frequency constant. The deuterium signal comes from the deuterium solvent used

22、 to prepare the sample. The animation window contains plots of the deuterium resonance lock frequency, the small additional magnetic field used to correct the lock frequency, and the resultant Bo field as a function of time while the magnetic field is drifting. The lock frequency plot displays the f

23、requency without correction. In reality, this frequency would be kept constant by the application of the lock field which offsets the drift.鎖場是NMR譜儀中一個單獨(dú)的部分，主要用來調(diào)整NMR諧振頻率，監(jiān)控氘信號的諧振頻率并改變Bo場以維持諧振頻率穩(wěn)定。氘信號用來準(zhǔn)備樣品。窗口中包含了氘諧振頻率的圖像，用來修正鎖場頻率的小磁場以及Bo場。鎖場頻率表現(xiàn)并不是對頻率的修正。實(shí)際上，該頻率壓迫保持恒定以修正場的漂移。On most NMR spectromete

24、rs the deuterium lock serves a second function. It provides the reference. The resonance frequency of the deuterium signal in many lock solvents is well known. Therefore the difference in resonance frequency of the lock solvent and TMS is also known. As a consequence, TMS does not need to be added t

25、o the sample to set reference; the spectrometer can use the lock frequency to calculate reference. 大多數(shù)NMR譜儀氘鎖定有第二種功能。它提供出參考數(shù)據(jù)。大多數(shù)鎖場溶劑中氘信號的諧振頻率是已知的，因此鎖場溶劑的諧振頻率的不同和TMS也是已知的。所以TMS不需要對樣品設(shè)置參數(shù)，譜儀可以利用鎖場頻率計(jì)算參數(shù)。 Shim CoilsThe purpose of shim coils on a spectrometer is to correct minor spatial inhomogeneities

26、 in the Bo magnetic field. These inhomogeneities could be caused by the magnet design, materials in the probe, variations in the thickness of the sample tube, sample permeability（滲透性）, and ferromagnetic materials around the magnet. A shim coil is designed to create a small magnetic field which will

27、oppose and cancel out an inhomogeneity in the Bo magnetic field. Because these variations may exist in a variety of functional forms (linear, parabolic, etc.), shim coils are needed which can create a variety of opposing fields. Some of the functional forms are listed in the table below. 墊片線圈的作用是修正B

28、o場的空間不均勻性。不均勻性可能是以下因素造成：磁體設(shè)計(jì)，探頭材料，樣品管的厚度變化，樣品滲透性，以及磁體周圍的鐵磁材料。墊片線圈就是產(chǎn)生一個抵消不均勻性的小磁場。而可能存在一維的，二維的等不同形式的偏差，所以墊片線圈也需要能產(chǎn)生多樣的場。Shim Coil Functional FormsShim Function Z0 Z, Z2, Z3, Z4, Z5 X, XZ, XZ2, X2Y2, XY, Y, YZ, YZ2XZ3, X2Y2Z, YZ3, XYZ, X3, Y3By passing the appropriate amount of current through each c

29、oil a homogeneous Bo magnetic field can be achieved. The optimum shim current settings are found by either minimizing the linewidth, maximizing the size of the FID, or maximizing the signal from the field lock. On most spectrometers, the shim coils are controllable by the computer. A computer algori

30、thm has the task of finding the best shim value by maximizing the lock signal. 在每個線圈中通過一定量的電流便可以產(chǎn)生均勻的Bo場。 理想的電流設(shè)置通過最小化線寬，最大化FID尺寸，或者最大化鎖場信號得出。一般譜儀這些都是由計(jì)算機(jī)控制的，通過最大化鎖場信號得到最佳的shim值。2. 探頭Sample ProbeThe sample probe is the name given to that part of the spectrometer which accepts the sample, sends RF en

31、ergy into the sample, and detects the signal emanating from the sample. It contains the RF coil, sample spinner, temperature controlling circuitry, and gradient coils. The RF coil and gradient coils will be described in the next two sections. The sample spinner and temperature controlling circuitry

32、will be described here. 樣品探頭是指接受樣品，給樣品傳送RF能量以及檢測樣品信號的部分。它包括RF線圈，樣品旋轉(zhuǎn)器，溫度控制電路和梯度線圈，這里先討論樣品旋轉(zhuǎn)器和溫度控制電路。The purpose of the sample spinner is to rotate the NMR sample tube about its axis. In doing so, each spin in the sample located at a given position along the Z axis and radius from the Z axis, will ex

33、perience the average magnetic field in the circle defined by this Z and radius. The net effect is a narrower spectral linewidth. To appreciate this phenomenon, consider the following examples. In picture an axial cross section of a cylindrical tube containing sample. In a very homogeneous Bo magneti

34、c field this sample will yield a narrow spectrum. In a more inhomogeneous field the sample will yield a broader spectrum due to the presence of lines from the parts of the sample experiencing different Bo magnetic fields. When the sample is spun about its z-axis, inhomogeneities in the X and Y direc

35、tions are averaged out and the NMR line width becomes narrower. 樣品旋轉(zhuǎn)器的作用是把樣品管可以繞它的軸旋轉(zhuǎn)。凈影響是狹窄的光譜寬度。為了得到這個現(xiàn)象，考慮下面的例子，包含樣品的圓柱管軸向截面。在非常均勻的Bo場中樣品將產(chǎn)生一個狹窄的光譜。當(dāng)樣品繞z軸旋轉(zhuǎn)時，X和Y方向的不均勻性就體現(xiàn)出來導(dǎo)致NMR線寬變窄。Many scientists need to examine properties of their samples as a function of temperature. As a result many instrum

36、ents have the ability to maintain the temperature of the sample above and below room temperature. Air or nitrogen which has been warmed or cooled is passed over the sample to heat or cool the sample. The temperature at the sample is monitored with the aid of a thermocouple and electronic circuitry m

37、aintains the temperature by increasing or decreasing the temperature of the gas passing over the sample. RF Coils 許多科學(xué)家需要檢驗(yàn)樣品與溫度的函數(shù)關(guān)系。因此許多儀器都包含低于或高于室溫的樣品溫度?？梢杂靡鸭訜峄蚶鋮s過的空氣或氮?dú)馔ㄟ^樣品來加熱或冷卻樣品。樣品溫度可以通過包含可以改變通過樣品的氣體溫度的熱電偶或電子電路實(shí)現(xiàn)。RF coils create the B1 field which rotates the net magnetization in a pulse sequ

38、ence. They also detect the transverse magnetization as it precesses in the XY plane. Most RF coils on NMR spectrometers are of the saddle coil design and act as the transmitter of the B1 field and receiver of RF energy from the sample. You may find one or more RF coils in a probe. RF線圈在脈沖序列中產(chǎn)生一個旋轉(zhuǎn)凈磁

39、化的B1場。由于他在XY平面的進(jìn)動也可以驗(yàn)測到橫向磁化。大多數(shù)NMR的RF線圈都是鞍形的并可以作為B1場的傳導(dǎo)器及RF能量的接受器。可以在一個探頭中找到一個或多個RF線圈。Each of these RF coils must resonate, that is they must efficiently store energy, at the Larmor frequency of the nucleus being examined with the NMR spectrometer. All NMR coils are composed of an inductor, or induc

40、tive elements, and a set of capacitive elements. The resonant frequency, , of an RF coil is determined by the inductance (L) and capacitance (C) of the inductor capacitor circuit. RF coils used in NMR spectrometers need to be tuned for the specific sample being studied. An RF coil has a bandwidth or

41、 specific range of frequencies at which it resonates. When you place a sample in an RF coil, the conductivity and dielectric constant of the sample affect the resonance frequency. If this frequency is different from the resonance frequency of the nucleus you are studying, the coil will not efficient

42、ly set up the B1 field nor efficiently detect the signal from the sample. You will be rotating the net magnetization by an angle less than 90 degrees when you think you are rotating by 90 degrees. This will produce less transverse magnetization and less signal. Furthermore, because the coil will not

43、 be efficiently detecting the signal, your signal-to-noise ratio will be poor. 每個RF線圈都應(yīng)該是諧振的，因?yàn)樗麄円行实脑诒粰z驗(yàn)的核子的拉莫爾頻率下儲存能量。所有的NMR線圈都是由一個傳感器件和一系列電容器件組成。諧振頻率是由LC電路的電感和電容決定的，對每個特殊的樣品都要進(jìn)行調(diào)試。一個RF線圈都有一個帶寬和特定的共振頻率范圍。當(dāng)在RF線圈中放入一個樣品時，樣品的導(dǎo)電率和介電常數(shù)就影響了諧振頻率。如果這個頻率和正在研究的核子的諧振頻率不一樣的話，線圈不會效率的建立B1場或者探測樣品信號。旋轉(zhuǎn)90度時其實(shí)你并沒有

44、真正旋轉(zhuǎn)到90度。進(jìn)一步說，由于線圈不能有效地探測到信號你所得到的信噪比將非常低。The B1 field of an RF coil must be perpendicular to the Bo magnetic field. Another requirement of an RF coil in an NMR spectrometer is that the B1 field needs to be homogeneous over the volume of your sample. If it is not, you will be rotating spins by a dist

45、ribution of rotation angles and you will obtain strange spectra. B1場的RF線圈和Bo場必須垂直。另一個要求是B1場在樣品體積上是均勻的。不這樣的話你旋轉(zhuǎn)的角度將有偏差并得到奇怪的光譜。Gradient CoilsThe gradient coils produce the gradients in the Bo magnetic field needed for performing gradient enhanced spectroscopy, diffusion measurements, and NMR microsco

46、py. The gradient coils are located inside the RF probe. Not all probes have gradient coils, and not all NMR spectrometers have the hardware necessary to drive these coils.The gradient coils are room temperature coils (i.e. do not require cooling with cryogens to operate) which, because of their conf

47、iguration, create the desired gradient. Since the vertical bore superconducting magnet is most common, the gradient coil system will be described for this magnet. 梯度線圈在Bo場中產(chǎn)生梯度以提高光譜，散射測量和NMR的顯微精度。梯度線圈位于RF探頭。并不是所有的探頭都有梯度線圈而且并不是所有的NMR譜儀都有必要的硬件來驅(qū)動梯度線圈。梯度線圈是在室溫下產(chǎn)生需要的梯度。由于垂直鉆孔超導(dǎo)磁體是最常見的，梯度線圈就在這種此題上描述。Assu

48、ming the standard magnetic resonance coordinate system, a gradient in Bo in the Z direction is achieved with an antihelmholtz type of coil. Current in the two coils flow in opposite directions creating a magnetic field gradient between the two coils. The B field at the center of one coil adds to the

49、 Bo field, while the B field at the center of the other coil subtracts from the Bo field. 設(shè)想一個標(biāo)準(zhǔn)的磁共振坐標(biāo)系，Bo在Z方向的梯度可以有一個反亥姆霍茲型線圈實(shí)現(xiàn)。兩個通以相反電流的線圈將產(chǎn)生一個磁場梯度。The X and Y gradients in the Bo field are created by a pair of figure-8 coils. The X axis figure-8 coils create a gradient in Bo in the X direction du

50、e to the direction of the current through the coils. The Y axis figure-8 coils provides a similar gradient in Bo along the Y axis. Bo場的X和Y方向的梯度是由一對8字形線圈產(chǎn)生的。X方向的8字形線圈依據(jù)通過其電流的方向產(chǎn)生一個Bo在X方向的梯度，Y軸同理。3. 脈沖編程器及射頻放大器包括頻率綜合器，放大器及有關(guān)的電子器件。通常一臺譜儀有若干通道，分別工作在不同的核的共振頻率上，在實(shí)驗(yàn)中可以同時施加作用于不同核的射頻脈沖。脈沖編程器控制脈沖的時序，長度，幅度，相位甚

51、至形狀。4. 接收器包括前置放大器，相敏檢波器，模數(shù)轉(zhuǎn)換器由于核磁信號很弱，前置放大器總是盡量靠近探頭，其性能決定了儀器能達(dá)到的信噪比相敏檢波器從射頻信號中檢出低頻的核磁信號（相當(dāng)于旋轉(zhuǎn)坐標(biāo)系中的信號），并完成正交檢波模數(shù)轉(zhuǎn)換器或稱ADC完成模擬信號的數(shù)字化，一般譜儀考慮到轉(zhuǎn)換速度和動態(tài)范圍，配置的為16bit ADC，其動態(tài)范圍為到即（-32,768 to 32,767），因此一方面送到ADC的信號不能超出這個范圍，否則要失真出現(xiàn)截?cái)嘈?yīng)；另一方面信號也不能太小，若小于0.5bit，則無法被確定地記錄，此時噪聲確定是否記錄，因此需要長時間累加，而且由于量化噪聲還會產(chǎn)生嚴(yán)重的基線畸變.Digi

52、tal Filtering 數(shù)字濾波Many newer spectrometers employ a combination of oversampling, digital filtering, and decimation to eliminate the wrap around artifact. Oversampling creates a larger spectral or sweep width, but generates too much data to be conveniently stored. Digital filtering eliminates the hig

53、h frequency components from the data, and decimation reduces the size of the data set. The following flowchart summarizes the effects of the three steps by showing the result of performing an FT after each step. 許多新型的譜儀都用過密采樣，數(shù)字濾波和批量舍去的方法來消除人為的影響。采樣過密可以得到更大的光譜和掃描寬度，但產(chǎn)生的大量數(shù)據(jù)不易于儲存。數(shù)字過濾淘汰了數(shù)據(jù)中的高頻部分，批量舍去

54、減少了數(shù)據(jù)設(shè)置的數(shù)量。下面的流程圖總結(jié)了三個步驟對結(jié)果的影響。Lets examine oversampling, digital filtering, and decimation in more detail to see how this combination of steps can be used to reduce the wrap around problem. 讓我們更細(xì)致的檢驗(yàn)過密采樣，數(shù)字濾波和批量舍去來看看到底是怎么實(shí)施的。Oversampling is the digitization of a time domain signal at a frequency mu

55、ch greater than necessary to record the desired spectral width. For example, if the sampling frequency, fs, is increased by a factor of 10, the sweep width will be 10 times greater, thus eliminating wraparound. Unfortunately digitizing at 10 times the speed also increases the amount of raw data by a

56、 factor of 10, thus increasing storage requirements and processing time. 過密采樣是比需要的采樣寬度而收集更過的信號。例如采樣因子是10則掃描寬度也將是10倍大。不幸的是存儲要求和采樣時間也將大量增加。Filtering is the removal of a select band of frequencies from a signal. For an example of filtering, consider the following frequency domain signal. Frequencies above fo could be removed from this frequency domain signal by multipling the signal by this rectangular function. In NMR, this step would be equivalent to